1. Field of the Invention
The present invention relates to an immersion type lithographic apparatus.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., including part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
It has been proposed to immerse the substrate in the lithographic projection apparatus in a liquid having a relatively high refractive index, e.g., water, so as to fill a space between the final element of the projection system, i.e., the bottom of the projection system, and the substrate, thus providing an example of an immersion type lithographic apparatus. This enables more accurate projections and imaging of smaller features since the exposure radiation will have a shorter wavelength in the liquid. The effect of the immersion liquid may also be regarded as increasing the effective numerical aperture number NA of the system and also increasing the depth of focus. Other immersion liquids have been proposed, including water with solid particles (e.g., quartz) suspended therein. Thus, a lithographic apparatus may be provided with a fluid provider which is arranged to provide the immersion liquid, or to keep the liquid in its place. The liquid may be flowing to avoid local heating.
The substrate, or the substrate and the substrate table, may be immersed in a bath of immersion liquid. An example of such an arrangement is disclosed in U.S. Pat. No. 4,509,852 which hereby is incorporated by reference in its entirety. Alternatively, the immersion liquid may be provided by a liquid supply system only on a localized area of the substrate and in between the final element of the projection system and the substrate using a liquid confinement system, the substrate generally having a larger surface area than the final element of the projection system. An example of such an arrangement is disclosed in International Patent Application No. 99/49,504 which hereby is incorporated by reference in its entirety. The liquid is supplied by at least one inlet on the substrate, preferably along a direction of movement of the substrate relative to the final element of the projection system, and the liquid is discharged by at least one outlet which may be connected to a low pressure source. Various orientations and numbers of inlets and outlets positioned near the periphery of the final element are possible. Further, a liquid supply system may be provided with a seal member which extends along at least a part of a boundary of the space between the final element of the projection system and the substrate table. The seal member is substantially stationary relative to the projection system in the XY plane though there may be some relative movement in the Z direction (the direction of the optical axis of the projection system). A seal is formed between the seal member and the surface of the substrate. Preferably the seal is a contactless seal such as a gas seal, which may further function as a gas bearing. An example of such an arrangement is disclosed in European Patent Application No. 03252955.4 which hereby is incorporated by reference in its entirety.
European Patent Application No. 03257072.3, which hereby is incorporated by reference in its entirety, discloses a twin or dual stage immersion lithography apparatus. Such an apparatus is provided with two stages for supporting the substrate. Leveling measurements are carried out with a stage at a first position, without the presence of an immersion liquid, and exposure is carried out with a stage at a second position, where an immersion liquid is present. Alternatively, the apparatus has only one stage.
In a known implementation of a lithographic apparatus making use of immersion as described above, the liquid supply system to provide the immersion liquid is guided with respect to the substrate and/or the substrate table by a gas bearing, such as an air bearing. The gas bearing provides for a flow of a gas in a gap between the liquid supply system and the substrate or substrate table. Effectively, the liquid supply system may rest on a layer of such gas which flows in the gap. By the use of such a gas bearing, a certain amount of safety is obtained as the gas bearing commonly provides for a minimum distance between the liquid supply system and the substrate or substrate table. Further, to avoid a level difference between a surface of the substrate and a surface of a surrounding part of the substrate table which surrounds the substrate, a selection mechanism may have been implemented to select a substrate on its thickness. A substrate having a thickness which is too large or too small may be denied access to the lithographic apparatus, as the processing of such a substrate in the lithographic apparatus might result in a level difference between the substrate and the surface of the substrate table surrounding the substrate, which could result in a crash of the liquid supply system against the substrate resp. the substrate table, when passing from the substrate to the substrate table or vice versa. Such a crash is likely to occur in case that a height difference between a surface of the substrate and a surface of the substrate table surrounding the substrate amounts to a height of the gap between the substrate resp. the substrate table and the liquid supply system, or more.